feat(worker): add device registration_id cleanup worker

Add DeviceCleanupWorker to periodically clean up stale device tokens that haven't
been used within the retention period. This prevents accumulation of invalid
registration_ids and maintains data hygiene.

- New DeviceCleanupConfig with enabled, retention_days (3), and interval_minutes (360)
- New repository methods DeleteStaleDevices and DeleteOldestActiveDevice
- Updated RegisterDevice to evict oldest active device when reaching limit
- Reduced MaxDevicesPerUser from 10 to 3 per user
- Worker integrated into app startup/shutdown lifecycle

internal/config/config.go

@@ -20,6 +20,7 @@ type Config struct {
 	RateLimit         RateLimitConfig         `mapstructure:"rate_limit"`
 	Upload            UploadConfig            `mapstructure:"upload"`
 	FileCleanup       FileCleanupConfig       `mapstructure:"file_cleanup"`
+	DeviceCleanup     DeviceCleanupConfig     `mapstructure:"device_cleanup"`
 	OpenAI            OpenAIConfig            `mapstructure:"openai"`
 	TencentTMS        TencentTMSConfig        `mapstructure:"tencent_tms"`
 	Email             EmailConfig             `mapstructure:"email"`
@@ -115,6 +116,10 @@ func Load(configPath string) (*Config, error) {
 	viper.SetDefault("file_cleanup.retention_days", 7)
 	viper.SetDefault("file_cleanup.interval_minutes", 360)
 	viper.SetDefault("file_cleanup.batch_size", 100)
+	// 设备 registration_id 清理默认值（3 天不活跃即清理）
+	viper.SetDefault("device_cleanup.enabled", true)
+	viper.SetDefault("device_cleanup.retention_days", 3)
+	viper.SetDefault("device_cleanup.interval_minutes", 360)
 	viper.SetDefault("s3.endpoint", "")
 	viper.SetDefault("s3.access_key", "")
 	viper.SetDefault("s3.secret_key", "")